Many of the conventional photoinitiators or photoinitiator systems are activatable primarily by radiation in the ultraviolet or shorter wavelength region (i.e., blue and green) of the visible spectrum. The availability of reliable, relatively inexpensive lasers which emit in the longer wavelength (i.e., red) region of the visible spectrum and which can be used as output devices for electronic imaging systems has made it desirable to develop photoinitiator systems which are activatable by this spectral region. Applications for photopolymerizable compositions containing these photoinitiator systems include graphic arts films, proofing, printing plates, photoresists, holograms, and holographic optical elements.
Photopolymerizable compositions containing 2,2', 4,4', 5,5'-hexaarylbisimidazoles, or HABI's, are well known. Sensitizers which extend the sensitivity of these compositions are disclosed, for example, in Baum, U.S. Pat. No. 3,652,275; Dueber, U.S. Pat. No. 4,162,162; Smothers, U.S. Pat. No. 4,917,977; and Monroe, U.S. Pat. No. 4,987,230. However, despite the advances which have been made, there is a continuing need for photoinitiator systems which are sensitive to the visible region of the spectrum.